TurboTax says my overpayment to 401K retirement account is $500 in 2019. But my own calculation gives me a different number. Which one should I use?

For 2019, if you are a W-2 employee, your total 401(k) contributions are limited to $19,000, plus an additional $6000 if you are 50 years old or older.   That includes both employer and employee money. 

The employer matching contributions are separate from the individual's $19,000 and $6,000 limits.  Each plan is separately subject to an overall limit of the lesser of $56,000 plus catch-up or the employee's compensation.  TurboTax only checks deferrals against the $19,000-plus-catch-up limit for amounts reported in box 12 of the individual's Forms W-2.